1. Field of the Invention
The present invention relates to a support for a lithographic printing plate, a presensitized plate, and a method of producing a lithographic printing plate. More particularly, the present invention relates to a support for a lithographic printing plate having an optimum surface shape achieving both high scum resistance and long press life when a lithographic printing plate is produced, a presensitized plate using the support for a lithographic printing plate, and a method of producing a lithographic printing plate excellent in scum resistance and press life.
2. Description of the Related Art
Lithography is a printing process which makes use of a nature that water and oil are essentially unmixable with each other. On the printing plate surface of a lithographic printing plate used in this process, areas that receive water and repel an oil-based ink (hereinafter referred to as “non-image areas”) and the other areas that repel water and receive an oil-based ink (hereinafter referred to as “image areas”) are formed.
Since an aluminum support for a lithographic printing plate used for a lithographic printing plate (hereinafter referred to simply as “a support for a lithographic printing plate”) is used as such that the surface thereof plays a role of non-image areas, various conflicting performances are required, such as excellency in water wettability and water receptivity as well as excellency in contact characteristics with an image recording layer provided on the support.
If the water wettability of a support is too low, an ink is attached to non-image areas at the time of printing, thereby causing a dirt on a blanket cylinder and further a so-called scum. In addition, if the water receptivity is too low, a shadow portion may be plugged unless much fountain solution is applied at the time of printing. Therefore, a so-called water range is narrowed.
In order to obtain a support for a lithographic printing plate excellent in these performances, generally asperities are provided for the surface of an aluminum plate by performing graining (graining treatment) thereon. As shown below, various shapes of the asperities are proposed. JP 8-300844 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”), describes a triple grained structure having a grained structure of large undulation, a grained structure of medium undulation, and a grained structure of small undulation, with aperture diameters of the grained structures of medium and small undulations specified. JP 11-99758 A and JP 11-208138 A describe specifying the diameter of a grained structure with small undulation in the double structure with a grained structure of large and small undulations. JP 11-167207 A describes an art providing large and small double concave portions (pits) and further fine protrusions JP 2023476 B (the term “JP XX-XXXXXX B” as used herein means an “examined Japanese patent publication”) describes a double structure with the aperture diameters specified. JP 8-300843 A describes a double structure with a factor a30 indicating smoothness of a surface specified. JP 10-35133 A describes a structure with the ratio of diameters of pits superimposed at a time of a plurality of electrochemical graining treatments (hereinafter referred also to as an “electrolytic graining treatment”).
Methods to be used for this graining include a mechanical graining treatments such as ball graining, brush graining, wire graining and blast graining, an electrolytic graining treatment performing electrolytic etching on an aluminum plate in an electrolyte containing hydrochloric aid and/or nitric acid, and a composite graining treatment combining a mechanical graining treatment and an electrolytic graining treatment as described in U.S. Pat. No. 4,476,006 and the like.